This invention relates to a demodulation device and, in particular, to a demodulation device applicable to microwave digital communication by the use of a multiphase phase modulation system and the like.
Various demodulation devices are known in the art. A first conventional one of the demodulation devices is disclosed in Japanese Patent Prepublication (Kokai or Publication of Unexamined Patent Application) No. 16049/1985 by Mizoguchi. As will later be described in detail in conjunction with the drawing, the conventional demodulation device is for demodulating a phase-modulated wave and comprises a baseband signal producing arrangement, a phase information producing circuit, and a wave producing arrangement. The baseband signal producing arrangement is supplied with the phase-modulated wave and with a reproduced carrier wave and is for producing a baseband signal having particular information which is dependent on the phase-modulated wave. The phase information producing circuit is responsive to said baseband signal for producing a phase difference signal representative of difference between the phase-modulated wave and the reproduced carrier wave. The wave producing arrangement is for producing the reproduced carrier wave in accordance with the control signal.
However, the conventional demodulation device often suffers from a phenomenon called a pseudo pull-in state such that a sync pull-in operation is carried out even when the reproduced carrier wave has a reproduced carrier frequency different from a carrier frequency of a reception signal, namely, the phase-modulated wave.
To solve the above-mentioned problem of the pseudo pull-in state, a proposal is made as a second conventional one of the demodulation devices in Japanese Patent Prepublication (Kokai or publication of Unexamined Patent Application) No. 91154/1993 by Sato and will later be described in detail in conjunction with the drawing.
However, the second conventional demodulation device inevitably has a complicated structure in order to cancel the pseudo pull-in state and to establish a normal phase synchronization in case where the pseudo pull-in state occurs. Specifically, the proposal must use two multilevel identifiers.